164 The Structure of Protoplasm 



when it is in the gel state is the key to this, and probably also to the 

 action of cilia, flagella, undulatory membranes, and muscles. 



Ameboid locomotion thus depends (a) upon a definite organiza- 

 tion of the cell or organism, namely, a superficial gel or viscous 

 layer and a central less viscous or fluid endoplasm, (b) upon the 

 contractile tension which protoplasm (a colloid) automatically exerts 

 when it gels, (c) upon local changes of viscosity: gel layer into 

 fluid or semifluid endoplasm, and endoplasm into gel layer, (d) 

 upon the regulation and the polarization of the viscosity changes, 

 and (e) upon a substratum. 



Ameboid type of locomotion depends upon a definite organiza- 

 tion of the cell or organism, namely, a superficial gel layer and a 

 central less viscous or fluid endoplasm. The necessity for this par- 

 ticular type of organization is evident if one considers any other hypo- 

 thetical type of organization. If gelation were central or immediately 

 about the central area and the nucleus and the fluid or less viscous 

 protoplasm superficial, ameboid locomotion would be impossible. 

 A uniform viscosity of the protoplasm from surface to center 

 throughout the organism or cell, whether in the sol, the gel, or a 

 semigel state would also preclude ameboid locomotion. 



Other important aspects of this organization are (a) the relative 

 and the absolute thickness of the gel layer and the endoplasm and 

 (b) the relative and absolute viscosity of the two layers. The thick- 

 ness and viscosity undoubtedly vary from moment to moment in 

 active cells and protozoa, less rapidly in inactive ones. They also 

 vary with the type and metabolic condition of the cell and the 

 organism. 



The factors which bring about this fundamental type of organ- 

 ization are unknown. Probably all cells and protozoa have either 

 permanently or temporarily this type of organization, even though 

 they do not migrate (plant cells and eggs, for example) or migrate 

 with the aid of special organs such as cilia, etc. All cells and 

 protozoa are derived from cells or protozoa that multiply by cleav- 

 age, and cleavage, as will be shown later, is dependent upon the gel 

 layer. Some types of cells become much modified and lose their 

 power to divide. Among them there may be some that lose this 

 general type of organization. Any theory that attempts to explain 

 the factors involved which produce this fundamental type of organ- 

 ization will probably be applicable to all cells and to protozoa. 

 Heilbrunn (1940) has a theory that the high viscosity of the cortex 

 (gel layer) depends on the presence of the calcium ion, and that the 



